That excuse is so 2005.
Like it or not, we went in because we thought - as far as I can tell, reasonably but wrongly - that Saddam Hussein was seeking and actually had WMD's. We also justified it on the grounds of creating a neo-conservative "city on a hill," believing ourselves to be on a mission from God to create a beacon of democracy in the Middle East. This "flypaper" theory didn't come into vogue until it was clear there were no WMD's, but that Saddam Hussein instead took advantage of the prestige the innuendo that he did gave him. The Flypaper theory, additionally, has been discredited by the London tube bombings and the countless stopped terrorist attacks here.
There's only one defense of the war necessary - with the best intelligence we had at the time, we reasonably believed Saddam had WMD's and so acted appropriately. Our mistake was not in going in, but trying to do it cheap so that the occupation was incredibly botched. Iraq is a quagmire, yes, but it is one because of actions that happened in 2004-2005, not 2003-2004.
The flypaper theory doesn't have a single credible counterterrorism analyst behind it. It's a political invention. It sounds good to people who don't understand militant Islam, but it's completely false. Most of the counterterrorism experts in the intelligence community was afraid that attacking Iraq would inflame tensions elsewhere, and lead to an increase in overall radicalization worldwide.
They were right. The only thing that's kept America safe in the interim is better intelligence cooperation between the various agencies, which was lacking before 9/11. While the idea of invading Iraq had some noble and worthwhile rationales, stopping terrorism was not one of them.
Islamic terrorism a complex problem that defies easy solutions like "kill em' there so we don't have to kill 'em here". It's a community-based social phenomenon, not a conventional military threat. Trying to apply a military solution to a social problem is bound to fail, as hindsight is bound to show us in a few years.
Check out the actual speeches getting authorization. WMDs was almost an afterthought. In the press it was and is, of course, the only cause because it is the one that did not pan out. Actually, it did and lots of stuff has been found but Bush refuses to talk about it. The discoveries are made over and over again then the press is allowed to belittle them and then deny them with no further reaction from the administration or the military or Bush allows as how well maybe they are not so big a deal. Bush has not supported the war to the people. He has allowed the enemy to monopolize the war in the media. Bush is, well, a Bush. He is his Daddy's son and seems to have the same lacunae in his vision and conceptualization that GHWB had. That is the primary reason that I would be very leery of a Jeb Bush candidacy much as I respect his term as governor in Florida.